Recently, energy-saving has been actively promoted to protect the environment. For battery-powered portable equipment, such as mobile phones, digital cameras, and the like, energy efficiency is especially important to prolong battery life. Such portable equipment widely uses a step-down type switching regulator that includes an inductor because it is efficient and can be made compact.
A known synchronous rectification type switching regulator generally includes a switching transistor and a synchronous rectification transistor, and operates in a continuity mode and a discontinuity mode. In the continuity mode, a current flows through the inductor continuously. By contrast, in the discontinuity mode, the current does not flow through the inductor continuously. Consequently, when the switching regulator operates in the discontinuity mode with a light load condition, a reverse current may flow from an output terminal to the inductor. As a result, the performance efficiency of the switching regulator decreases.
In the known switching regulator, when the reverse current occurs, the synchronous rectification transistor is shut off to create a shutdown state, thus preventing the reverse current. However, the switching regulator generally employs a current detection resistor to detect the reverse current, and consequently power efficiency decreases due to power loss at the resistor.
FIG. 1 illustrates a known step-down switching regulator that includes a reverse current detection function. In FIG. 1, the switching transistor includes a switching transistor Q101, a synchronous rectification transistor Q102, a comparator 123, and an inductor L101. The comparator 123 compares a voltage at a connection node of the switching transistor Q101 and the synchronous rectification transistor Q102 with a reference voltage Vref. When the switching transistor Q101 turns off, the synchronous rectification transistor Q102 turns on, during which condition a current is kept flowing through the inductor L101 because of energy that is stored in the inductor L101 while the switching transistor Q101 is on. A drain voltage of the synchronous rectification transistor Q102 drops to a negative voltage, that is, the current is kept flowing to an output terminal through the synchronous rectification transistor Q102 and the inductor L101. The energy stored in the inductor L101 decreases due to discharge of the charge stored in the inductor L101. Accordingly, the current flowing through the inductor L101 decreases.
When all the energy stored in the inductor L101 has been discharged while the switching transistor Q101 is off, the current flowing through the inductor L101 drops to zero and ultimately a reverse current begins to flow from the output terminal to the inductor L101. The drain voltage of the synchronous rectification transistor Q102 becomes a positive voltage. When the drain voltage of the synchronous rectification transistor Q102 exceeds the reference voltage Vref, an output signal of the comparator 123 is inverted to a high level. The output signal with the high level changes a gate voltage of the synchronous rectification transistor Q102 to a low level through a NAND circuit 124, shutting off the synchronous rectification transistor Q102. Accordingly, occurrence of the reverse current is then prevented.
In the known switching circuit, the reference voltage Vref is determined to have a temperature dependence so as to cancel temperature characteristics of on-resistance of the synchronous rectification transistor Q102. However, the synchronous rectification transistor Q102 is shut off after the reverse current flows. Accordingly, the power loss cannot be made zero and the performance efficiency of the switching regulator decreases as a result.
Another step-down/step-up switching circuit is proposed as shown in FIG. 2. The switching circuit 100 includes switching transistors SW101 and SW103 and synchronous rectification transistors SW102 and SW104 for step-down and step-up operations, respectively. Depending on an input voltage Vin, the switching circuit 100 performs the step-down operation or the step-up operation. However, it is not possible to prevent occurrence of a reverse current in the step-up operation if a voltage of the synchronous rectification transistor SW102 for the step-down operation is checked in the same way as is done in the switching circuit shown in FIG. 1.
The inductor current flowing through the inductor L101 is checked when the switching transistors SW101 and SW103 are both off. Variation of the inductor current ΔiL is now described.
ΔiL=Vout/L, in the step-down and step-down/step-up operations, and
ΔiL=(Vout−Vin)/L in the step-up operation, where L is an inductance of the inductor L101.
In the step-down operation, under a condition in which the switching transistor SW103 for step-up operation is off and the synchronous rectification transistor SW104 for step-up operation is on, the switching transistor SW101 and the synchronous rectification transistor SW102 for the step-down operation are switched on/off complementarily. In the step-up operation, under a condition in which the switching transistor SW101 for step-down operation is on and the synchronous rectification transistor SW102 for step-down operation is off, the switching transistor SW103 for the step-up operation is switched on/off complementarily to the synchronous rectification transistor SW104 for the step-up operation.
By contrast, in the step-up/step-down operation, the switching transistor SW101 for step-down operation and the switching transistor SW103 for step-up operation are turned on/off simultaneously. Further, the synchronous rectification transistor SW102 for the step-up operation and the synchronous rectification transistor SW104 for the step-up operation are turned on/off simultaneously. Furthermore, the switching transistor SW101 for the step-down operation and the switching transistor SW103 for the step-up operation are turned on/off complementarily to the synchronous rectification transistor SW102 for the step-down operation and the synchronous rectification transistor SW104 for the step-up operation.
Accordingly, in the step-down and step-down/step-up operations, the variation of the inductor current through the inductor L101 ΔiL is a fixed value if the output voltage Vout is constant. Further, the value of the inductor current variation ΔiL is different from the inductor current variation in the step-up operation. Further, it is found that the variation of the inductor current ΔiL in the step-up operation depends on the input voltage Vin. However, deviation of the input voltage Vin is not considered in the known switching regulator, when the reference voltage applied to the comparator for detecting the reverse current is determined. Consequently, an appropriate reference voltage is not determined for the step-up operation. As a result, a reverse current may occur and the performance efficiency of the switching regulator may decrease.